1. Field of Invention
The present invention relates to the study of viscoelastic properties of material at the nanoscale.
2. Background of Art
Viscoelastic properties of materials are currently measured with the help of dynamical mechanical analysis (DMA). DMA machines are dealing with the bulk properties at the macro level. DMA mode is also implemented for several nanoindenters (for example, manufactured by Hysitron (NanoDMA) and Agilent (DCM) http://www.hysitron.com/page_attachments/0000/0629/nanoDMA_Nanoscale_Dynamic_Mechanical_Analysis_for_Viscoelastic_Materials.pdf; http://cp.literature.agilent.com/litweb/pdf/5990-4211EN.pdf). The claimed ability to measure viscoelastic properties of polymers and biomaterials at the nanoscale might be confusing. While term “nanoscale” means indeed nano for hard materials, it can hardly be applied for soft materials if one is speaking about the lateral resolution.
As an example, using a standard Berkovich probe, a reliable contact can be reached starting from an indentation of ˜50-100 nm. This corresponds to the lateral indentation size of a micron. For some soft materials, like PDMS (the Young's modulus is about 0.1-10 MPa), a stable contact can be reached only by using a very dull spherical probe. The lateral resolution in that case could reach tens of microns.
When doing DMA measurements of polymers (polyurethane, polystyrene, PNA, etc.), the fastest time of the measurements is 2-3 minutes per single point of the surface. As previously indicated, soft polymers may not withstand such a long measurement time. Finally, it is unrealistic perform reasonable mapping of a surface. As an example, a rather modest resolution surface map of 128×128 pixels or points (taking an optimistic 2 min per point), would require almost 550 hours or more than 22 days of a continuously running instrument. Due to the time needed, this is impractical.
There are known way to accelerate measurements involving different frequencies. The methods for simultaneous multi-frequency measurements have been previously used to accelerate measurements in electrochemistry (time resolved Fourier transform electrochemical impedance spectroscopy (FT-EIS) (Garland et al., 2004, Popkirov and Schindler, 1993)), infrared (Ferraro and Krishnan, 1990, Urban and Mcdonald, 1990), NMR (Vandenboogaart et al., 1994, Kauppinen and Partanen, 2001) spectroscopy, and in the study of rheology of complex “rheokinetic” liquids (Fourier Transform Mechanical Spectroscopy) (Huang and Wen, 1994, Malkin, 2004, Wilhelm et al., 1999, In and Prud'hornme, 1993, Holly et al., 1988) (Kulichikhin et al., 1984, Malkin, 1987).
Jesse et al. (U.S. Pat. No. 7,775,086) teach the method of bandgap excitation applied to the atomic force microscopy. Multiple frequencies are generated by a pulse-like signal of a finite duration having finite and predefined amplitude and phase spectrum in a given frequency band(s). The amplitudes are significant only in a rather narrow range (band) of frequencies around the chosen one. As a result, the method of Jesse et al. allows obtaining information in the narrow band around the chosen frequencies. All examples taught by Jesse et al. do with relatively high frequency started from 5000 Hz up to several hundreds of KHz.
Sokolov (U.S. Pat. No. 7,761,255) teaches the use of atomic force microscopy method to study dynamic properties of soft materials utilizing simultaneous multi-frequency measurements, and therefore, accelerating the measurements.
A traditional AFM indentation method is based on force and depth curve analysis (Pethica et al., 1983, Oliver and Pethica, 1989, Oulevey et al., 2000a, VanLandingham et al., 2000, VanLandingham et al., 2001, Oliver and Pharr, 2004). An extension of the technique was proposed to measure the frequency specific indentation (Lucas et al., 1997, Oulevey et al., 2000b, Herbert et al., 2008b, Hou et al., 2006) in a regular nanoindentation manner (one frequency at a time). Recently, two new high-resolution high-speed rigidity mapping AFM techniques have been developed by Veeco (HarmoniX™ and PeakForce™) (Sahin et al., 2007). These methods allow measuring rigidity modules, at, for example, 512×512 surface points, as fast as 15-30 min. Both techniques use a rather high operational (single) frequency (tens and hundreds of kHz for HarmoniX and ˜1 KHz for PeakForce). The typical range of interesting frequency dependence, however, is substantially lower (10-300 Hz used by Hysitron). Thus, the high-resolution, high-speed, multi-frequency viscoelastic mapping has not been disclosed as of yet.
3. Objects and Advantages
It is a principal object of the present invention to provide an apparatus and method for collecting data and assisting in the performance of high resolution, high-speed, multi-frequency viscoelastic mapping.
Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter.